Improve getting started documentation (specifically for OSX)

[bradleypeabody]

I tried getting up and running with PropWare on OSX (following http://david.zemon.name/PropWare/#/getting-started ) and was able to get things working; but there are two important points which I basically had to just guess as I didn't see them covered.

The issue is that the build setup does not appear to know where to look for either the PropWare libraries, or the propeller-gcc binaries (and for the latter the error message is somewhat misleading because instead of finding propeller-gcc it finds clang and then complains about how some of the options being passed to the compile command are wrong... because it's the wrong compiler).

Setting these two environment variables before running 'cmake -G "Unix Makefiles" ..' solved it:

export PROPWARE_PATH=~/Downloads/PropWare/
export GCC_PATH=/Applications/SimpleIDE.app/Contents//propeller-gcc/bin/

It may be that I just don't know where to put these two items on my system and possibly including that information would help also. But I think either way making it clear that these are the environment variables that control where to find these items, with an example, would be very helpful for the next person who comes along and tries to do what I just did.